Those skilled in the relevant art are cognizant of a variety of devices intended to be surgically implanted in a human patient to brace or otherwise affect the vertebral column. Many of these devices are adapted to fasten a series of vertebrae together by means of screws, clamps, and connective members. Examples include Lumb et al., U.S. Pat. No. 3,648,691; Crock et al., U.S. Pat. No. 3,997,138; McKay et al., U.S. Pat. No. 4,003,376; Hall, U.S. Pat. No. 4,041,939; Nissinen, U.S. Pat. No. 4,078,559; Miskew, U.S. Pat. No. 4,274,401; Dunn, U.S. Pat. No. 4,289,123; and Tanner, U.S. Pat. No. 4,361,141.
In all such devices, it is necessary to anchor the device to vertebrae in a way that is strong and secure and yet is not damaging to the vertebrae. Furthermore, it is desirable to minimize trauma generally and to guard against damage to the spinal cord running up through the vertebral column. These goals are accomplished in various ways in the devices referred to. In addition, in certain surgical procedures it is conventional to wire vertebrae, sometimes by threading wire beneath the vertebral lamina and carrying the wire back over the vertebral lamina before extending the wire to the next vertebra. It will be apparent that this process involves risk to the spinal cord located in the spinal foramen. Some devices are designed to engage individual vertebra with hooks that hook over the edge of the lamina without extending appreciably into the spinal foramen, thus minimizing risk to the spinal cord. Miskew discloses an example of such a device, as is the device shown in K. Roosen, A. Trauschel, and W. Grote, "Posterior Atlanto-Axial Fusion: A New Compression Clamp for Laminar Osteosynthesis," Arch. Orthop. Traumat. Surg. 100: 27-31 (1982).
In addition to providing for the secure clamping of such a device to the vertebrae, it is also desirable that the device be adjustable so that a single device may be adjusted to individual variations in the dimensions and relative locations of vertebrae in any given patient. In particular, when a device is being implanted to stabilize cervical vertebra injured traumatically or otherwise, adjustability of the device to meet the relative orientation and distance between the vertebrae is required.
With special reference to the surgical stabilization of fractures of cervical vertebrae, not only is it commonly necessary to stabilize adjacent vertebrae relative to each other, but it is also frequently necessary to bridge a damaged vertebra. For example, it might be found necessary to attach and stabilize the relationship between the third and fifth cervical vertebrae in such a way as to prevent the application of compressive pressures to the fourth cervical vertebra. Such stabilization is conventionally done with wire wrapped around various portions of the vertebrae and then tightened down. Additionally, bone grafts may be utilized to provide spacing and long-term support. Acrylic surgical cements mixed at the time of surgical intervention and hardened in place likewise may be used to provide support and set spacing and relative orientation. The wire that is used presents the dangers referred to above when it is passed underneath the laminae. In addition, several wires commonly are used to fasten the laminae together. Because the relative tension of each wire is only subjectively estimated, it is possible for such wires to break because of excess loading, requiring additional surgical intervention.